Speed vs Turbulence

[G-V]

Reduce indicated airspeed by 25% = Reduce effect of turbulence by 50%

True or not true?

[Sidestick_n_Rudder]

The g loads from vertical gusts are linear with speed IIRC, so reduce speed by 25% - reduce effect of turbulence by 25%.

However, you can't reduce your speed by 25% in a jet transport In cruise...

[mustangsally]

I tend to lean toward a very strong FALSE. At economical cruise a 25% reduction in IAS would put a whole lot of aircraft at just about the pre-stall buffet margin or worse. The best option for is to avoid the turbulence. If unable, maintain turbulent penetration speed as published for the airframe at given altitude. Disengage altitude hold and allow some variation in altitude. Frequently a strong updraft is followed by a strong down draft. Remember to let the ATC know what is happening.

[G-V]

Of course you can reduce airspeed by 25% in cruise if you are cruising at FL250 due to ATC restriction.

Anyway, the topic is not how to avoid turbulence or what to do if you are experiencing turbulence.

Let me make it easier:

Reduce indicated speed by 0.1% = Reduce effect of turbulence by 0.2%

True or False?

@Sidestick: Do you have any reference? what about horizontal gusts?

[Sidestick_n_Rudder]

Depends on what kind of airplane you are talking about. In a jet transport in cruise altitude reducing speed by 25% would get you into the low speed buffet/stall region.

Horizontal gusts don't really produce g forces, the main problem is airspeed excursions up, or down, in extreme cases outside of the flight envelope. That's why turbulence penetration speed is somewhere in the middle of the operating speed range, to keep maximum margin to both the low and high speed limits.

As for the vertical gusts - I don't have any reference, but it is basic aerodynamics and some geometry. G-load in a gust is proportional to the square of the speed and to the change of the AoA.

The change in the AoA in turn is proportional to the magnitude of the gust and inversely proportional to the airspeed.

If you combine the two together, the net effect of airspeed will be linear - i.e. twice the speed, twice the g'load for the same gust.

[wiedehopf]

you might want to give this a read:
http://www.nar-associates.com/technical-flying/turbulent/TurbulentPenetration_wide_screen.pdf

the available lift increases with speed squared.

if we assume that the turbulence increases the angle of attack a non-speed-dependent amount additional lift due to turbulence is reduced proportional to speed squared.

now at a higher speed the absolute angle of attack increase due to a change in vertical windspeed (updraft) is less.
so our previous assumption is not very helpful and i'd say the updraft induced additional lift is proportional to speed.

but and this is the big thing: the maximum available lift at the stall angle of the wing is proportional to the speed squared.
therefore if your wing temporarily stalls due to an updraft the total lift available increases with the speed squared.
the additional lift due to the updraft decreases even more dramatically approaching stall speed.

but as the linked pdf describes you normally fly a comfortable speed at which the wing at maximum AOA can't rip your wing apart and you are still fast enough for good controllability.


EDIT:
reading the other pdf linked ..... what i linked seems like it would not apply to jet transports :)
stalling might be good if it's the only way to keep your wing from falling apart but the accompanying loss of control is really bad.

[Capn Bloggs]

Well that discussion speared off on a tangent pretty quickly.

I reckon it's 25% gives 50% more bump ie the V2 thingy. But I think I said that in an old thread here on Prune and somebody shot me down. All I know is that if I slow down in the bumps, the bumps get less.

[B2N2]

https://www.google.com/patents/US4796192

Haven't read it so I'll store it here for later

[maui]

Mustangsally.
My reading of limitations is that Turb Penetration Speed is for severe turb. Check the definition of severe. In my book it requires things to be lifting of the floor. Too often seen speed reduced unnecessarily, reducing the low speed margin to uncomfortable levels.
maui

[PEI_3721]

maui, severe turbulence only.
Maybe, but how will you know if the turbulence yet to be encountered is severe, or for that already encountered, then it's probably too late to change speed.

The AFM turbulence speed is just that; and why all this concern about low speed margin?

[misd-agin]

Mustangsally.
My reading of limitations is that Turb Penetration Speed is for severe turb. Check the definition of severe. In my book it requires things to be lifting of the floor. Too often seen speed reduced unnecessarily, reducing the low speed margin to uncomfortable levels.
maui



Test pilot at one of the two largest airliner manufacturers - "we laugh at you airline guys who slow down for every bump. It's for turbulence penetration and not the stuff you experience every day."

[misd-agin]

maui, severe turbulence only.
Maybe, but how will you know if the turbulence yet to be encountered is severe, or for that already encountered, then it's probably too late to change speed.

The AFM turbulence speed is just that; and why all this concern about low speed margin?



Severe turbulence encounter is very, very, rare. I can recall on in over 20,000 hrs.


We've prepared for severe turbulence more often but actual encounters is very rare.

[misd-agin]

You're not slowing an airliner's KIAS by 25% at cruise.


Speed range ('window') between low speed 1.2 margin and redline at cruise altitudes is frequently 30-50 kts. Mach changes of .01 is a small amount of indicated airspeed (3-5 KIAS). KIAS is 250-300 KIAS. That means the 'window' speed range is approx. 15-20% of KIAS.


Actual cruise speed is typically about the mid point, or slightly faster, of the 'window'. So typical cruise KIAS is about 15-25 KIAS above the 1.2 yellow band ('hook'). So the speed available to reduce by, while staying above the hook, is maybe 9-12% of KIAS.


KIAS speed reduction from typical cruise to 'turbulence penetration speed' is more like 1-2% of KIAS.

[G-V]

The AFM turbulence speed is just that; and why all this concern about low speed margin?


Because pilots appear to be illiterate. The vast majority cant understand that the topic of this discussion is not how to avoid turbulence or how to operate transport or not transport cat airplanes, or about the stall or slow flight etc.

ITS A THEORETICAL AERODYNAMICS QUESTION! :ugh:



Reduce indicated speed by 0.1% = Reduce effect of turbulence by 0.2%

True or False?



I have to admit, that it is a very complex topic and only few people here will have a good grasp of this matter and majority will just guess (including myself), but I am still hoping that something good will come out of it.

[G-V]

You're not slowing an airliner's KIAS by 25% at cruise.



One more of this kind.

[G-V]

People, please don't assume that it is all about your operation, or your type etc. It is not.

For example, in business aviation its not about the structure of the airplane, but instead it is about keeping your job. Many owners are very nervous flyers and fire people for bumps.

Edit:
The change in the AoA in turn is proportional to the magnitude of the gust and inversely proportional to the airspeed.

If you combine the two together, the net effect of airspeed will be linear - i.e. twice the speed, twice the g'load for the same gust.



but and this is the big thing: the maximum available lift at the stall angle of the wing is proportional to the speed squared.
therefore if your wing temporarily stalls due to an updraft the total lift available increases with the speed squared.
the additional lift due to the updraft decreases even more dramatically approaching stall speed.


Please more of that.

[scifi]

Don't know where you should fit this in, but stall speed is proportional to square root of G load. Such that you can't stall at zero G ( when everything is floating in the cabin.)
Conversely if you encounter a +2G event, your stall speed will be 1.414x more.

[lomapaseo]

ITS A THEORETICAL AERODYNAMICS QUESTION!

and here I was thinking it was a force-deflection issue.

I had thought that the idea was to penetrate it with the primary vector being along the intended line of flight.

So to minimize excursions up or down or sideways, don't slow down in it.

Of course this presumes that the aircraft has been designed and certified for the level of turbulence forces.

[Capn Bloggs]

Test pilot at one of the two largest airliner manufacturers - "we laugh at you airline guys who slow down for every bump. It's for turbulence penetration and not the stuff you experience every day
Dear Test Pilot, we slow down to make the ride nicer for the FA and pax. :rolleyes:

[goeasy]

That's rubbish. Slowing down doesn't improve the ride, just prolongs your time in the turbulence.... the test pilots have it right. Turbulence penetration speed is for a category of turbulence that occurs very very rarely.

As far as I have read, nearly all the major severe turbulence occurrences have occurred in clear air With little, or no, prior turbulence to slow down for. But nearly everyone slows down at first bump, for no other reason than they see everyone else do it. Sheep operating procedure!

[G-V]

That's rubbish. Slowing down doesn't improve the ride, just prolongs your time in the turbulence....

IMHO your post is rubbish Goeasy. it is clear that penetrating the same pocket of turbulence at a higher speed will cause higher G-loads.

[Capn Bloggs]

Slowing down doesn't improve the ride, just prolongs your time in the turbulence
OK, next time you see that nice Cu in front, enter it at 320. Then do an orbit and hit it at 250. Tell how you get on. Oh, and ask the cabin crew what they thought about the rides.

[Sidestick_n_Rudder]

Yeah, but if you slow down from Ma 0.82 to Ma 0.8, you change the speed by what - 5 kts? How's that supposed to make a difference?

[oicur12.again]

Yeah i always get a chuckle when lads reduce from .8 to .79 or .78 in turbulence. Makes zero difference to the ride down the back.

[goeasy]

G-V and capt bloggs.... unlike you I'm not stupid enough to go through CB's. I'm referrring to those pilots in clear air who chose to slow up from .82 or 81 to .8 and truly believe they are improving the ride. Totally unprovable, and highly unlikely. Just sheep!

Even in severe turbulence the Airbus QRH only says 'CONSIDER' reducing to Turbulence penetration speed as specified in the table. It isnt even mandatory. Why?

[PEI_3721]

Turbulence speed is not about comfort, the ride down the back; it is required to protect the aircraft structure… and you!
If a manufacturer advises ‘consideration’ then consider… what if. Flying, airmanship, sensible interpretation does not have to be mandated.

G-V see CS 25.341 (and AMC) which discusses turbulence loads; noting that ‘load’ is force, which involves mass and acceleration. Some of the equations might relate to the discussion on a change speed re the effect of turbulence on structure or ride.
Also there are references in certification to ‘extreme’ turbulence, something above severe, but as yet I cannot find a definition which might be suitable for crew use.

[RVF750]

Personally, I see no problem slowing a bit to put yourself equally in between the two limits if it makes you feel better. In the B737NG I do notice the ride is better at the published speed for bumps .76/280kts. Perhaps it works better for the wings acting as suspension? Simply that the flex in the wings works better than when slower

[underfire]

The basic assumption is that the wind turbulence is horizontal. That is where the issues come from.

Consider gusting conditions that have a vertical factor, not simply the resultant wind component.

You have level flight with the associated AoA, and encounter a vertical gust. (be it uplift of downdraft) The effect from vertical winds on AoA can be significant. This is why slowing down appears to lessen the effect.

Given the multitude of angles that a vertical wind can come from, it seems that there really cannot be a definitive direct correlation between speed reduction and the effect on the G force componenet.

https://cdn.avweb.com/media/newspics/325/p1aeamfkh1gnu39i17ls1mn1pgc8.jpg

Consider that winds are flowing like currents in a stream, winding horizontally and vertically, or ocean waves (kelvin-helmholtz waves are an extreme example) but vertical rolling winds happen on a mesoscale all the time.

http://www.toptenz.net/wp-content/uploads/2011/10/The-Kelvin-Helmholtz-Wave-Cloud-by-richardjs1.jpg


So, a few years ago, the FAA had some guidance for small aircraft operators...given that the focus was on G acceleration, you may find some value in these equations.

VA: manuvering airspeed
VB: turbulence penetration speed (max speed where gust will not overstress the ac)

VA and VB are derived as a function of VS, clean stall speed:
VA (gross weight) = 1.95 × VS VB (gross weight) = 1.6 × VS
VA (current weight) = VA (gross weight) × √(current weight ÷ gross weight

(note: VB is slower than VA)

Currently, the FAA and other groups are looking at the circular velocity of the wake vortex, and how to convert this to G force and the associated effect per aircraft type, but that is a long ways off.

[Capn Bloggs]

G-V and capt bloggs.... unlike you I'm not stupid enough to go through CB's.
Who said anything about going thru CBs?

The fact is going slower through turbulence improves the ride. I reckon it's because L=V².

[goeasy]

Bloggs I don't disagree with that point whatsoever. My argument is that the significant speed reduction required to appreciably reduce turbulence effect is not 0.01 or 0.02mach and just isn't available in the confines of coffin corner.

But many pilots seem happy to reduce to or below best l/d speed (green dot on bus) just to 'lighten the load' on the wing. As opposed to trying to maintain mid-point between green dot and red line for max protection from both limits. Call it common sense or airmanship?

Neither is very common now!

[G-V]

OK, next time you see that nice Cu in front, enter it at 320. Then do an orbit and hit it at 250. Tell how you get on. Oh, and ask the cabin crew what they thought about the rides.

G-V and capt bloggs.... unlike you I'm not stupid enough to go through CB's.

@goeasy
Before you come here and start trolling I suggest:

1. Learn how to read
2. Learn how to understand what you are reading and then, may be, you will be able to grasp what this thread is about.
3. Study aviation language / METAR code and, may be, you will learn the difference between Cu and CB

I will try to explain to you what capt bloggs communicated quite clearly:
If you fly thru the same pocket of turbulence (Cu for example) at 320 knots, you will experience much stronger turbulence than doing so at 250 knots.
That is a common fact. Most professional pilots are in agreement about it as you mentioned.

Now the purpose of this thread is to discuss if G-loads decrease linear to the airspeed or not. Nothing else.
It is not about reducing your Mach from 0.82 to 0.80.
At FL250, for example,you can maintain 320 KIAS or slow down to 250 KIAS.

It is clear that you cant contribute anything to this topic so please stop trolling.

[G-V]

Yeah, but if you slow down from Ma 0.82 to Ma 0.8, you change the speed by what - 5 kts? How's that supposed to make a difference?

This 5 knots could make a difference between life and death.

[G-V]

Turbulence speed is not about comfort, the ride down the back; it is required to protect the aircraft structure… and you!
If a manufacturer advises ‘consideration’ then consider… what if. Flying, airmanship, sensible interpretation does not have to be mandated.

G-V see CS 25.341 (and AMC) which discusses turbulence loads; noting that ‘load’ is force, which involves mass and acceleration. Some of the equations might relate to the discussion on a change speed re the effect of turbulence on structure or ride.
Also there are references in certification to ‘extreme’ turbulence, something above severe, but as yet I cannot find a definition which might be suitable for crew use.

I checked § 25.341 Gust and turbulence loads.
Couldn't find anything about relationship of G-loads and speed.

[PEI_3721]

G-V, speed - g relationship, is more by inference than exact value.
A very, very simple view of CS 25;-
A value of g is chosen - a structural limit. The aircraft response to the gust model is checked for a range of speeds, reducing until an acceptable compromise if found - the speed at which g is within the structure limit.
A similar assessment is made for low speed margin, with speed increasing, the result of the two being the turbulence speed or range of speeds. As per #28.
Whether this indicates a V^2 or linear relationship with force depends on the equations - a simple view might favour V^2.

The thread question relates to the effect of turbulence, presumably relating to comfort.
Comfort is subjective, an individual assessment. What is rough for a passenger could be a non event for the crew. Similar assessments might depend on aircraft type, or differences in structure and materials. At least one type that I flew had different responses between the cockpit (and rear cabin), and the centre section being least affected (flexing bent-banana mode), and again different modes - sway vs bounce which can affect people differently. And then there is roll …

Irrespective of the degree of comfort, passengers appear to favour a safe arrival, then being able to share their ‘horrific’ experiences, than to suffer physical harm or even not to arrive at all.

[underfire]

https://cdn.avweb.com/media/newspics/325/p1aeamfmiepon19m7195g11ik1ro19.jpg

[Yaw String]

Thread creep again...but,I think many of us,up there,like to see our fellow pilot opening the speed window,and reducing speed,when the going gets slightly more than rough.
For instance,when going for the suckers gap,that so often closes up,it gives us the feeling that our fellow pilot is thinking in the right direction.
When I sit in the tops(if I can't avoid them), and my colleague does nothing with the speed window,as the Mach increases to .87 or .88, I find myself having less than respectable thoughts.
As many have pointed out here,the above scenario will,99.9% of the time,not result in severe turbulence....but,that is not how we operate,is it!
We hope for the best,and plan for the worst...

[Stan Woolley]

When I sit in the tops(if I can't avoid them), and my colleague does nothing with the speed window,as the Mach increases to .87 or .88, I find myself having less than respectable thoughts.

Hi Yaw String

Thankfully I'm past it all,which is probably just as well but I'd hope that an ex-national man would do a bit more that have 'less than respectable thoughts' I'd kick ass and tell him to slow the ----er down!

CRM anyone? :8

:ok:

[macdo]

330 def. rides bumps better at .80 than at the wing design cruise mach of .82 and I always do this with respect to the margin available for pax comfort, the Athr also seems to cope better.
I never found that slowing down made much difference on the 320.

[goeasy]

In fact its always quite a good discussion topic. What I find curious about some claims here, is how can anyone say convincingly... oh by reducing speed from X to X made the ride better? By nature turbulence isn't constant, so speed reduction may have been followed by less turbulent air. Couldn't it?

What if you reduced speed then turbulence coincidentally increased?

That is all I was trying to emphasise, that we have no idea if a reduction of 5-10 knots actually improved the ride, without turning back through the same turbulent area, and comparing G-loads. I'm quite happy to accept that a reduction of speed by 20-30 knots or more will likely lighten the loads, but this just isn't feasible at the cruise altitudes of most airliners these days.

In my book, staying as close to mid-point between two red lines may be the safest option, in anything stronger-than-light turbulence.

[FE Hoppy]

Fly turb penetration speed. If you don't have one published look at the manoeuvre capability graph and fly the speed that coincides with the peak of the curve for your altitude. This gives you the most manoeuvre margin and probably matches the turb penetration speed you forgot!!

[G-V]

@Goeasy

I agree with 99% of what you just said. There is no way of knowing how slowing down affected the ride unless we fly thru the same (theoretically and practically impossible) pocket of turbulence 2 times at two different airspeeds.

No question, at optimum cruise altitudes we cant slow down significantly. Sometimes not at all.
In my airplane, if we fly at optimum altitude .80 above FL430, we are already at Vref or very close to it so there is no margin to slow down anyway.

However, if you depart Shanghai to Tokyo, Chinese will keep you at FL260 for 250nm. That is where we have an option to fly 320 knots or as slow as 240 knots at moderate weights and still have safe margins to stall (like Vref +70 or so).

@FE Hoppy

In my airplane turb pen speed is 240 below 10000ft and 270/.80 above
It doesnt take a rocket scientist to realise that it doesnt work like that: 240 knots at 9999ft and 270 knots at 10001ft.

For me personally, if I have a very good margin to Vref at current configuration and altitude with PAX, I will slow down below 270 KIAS for light to moderate and above. If it is ferry flight, I normally never slow down below Vb.

[G-V]

After reading all the input, it appears that if turbulence imposes lift changes (ie vertical wind shear or horizontal turbulence which comes on the nose or tail) than its:

increase speed by 1% = increase bumps by 2%

If it is horizontal wind shear from 3 or 9 O'clock, than its linear as there is no lift change involved:

increase speed by 1% = increase bumps by 1%

Comments?

[FE Hoppy]

@Goeasy

I agree with 99% of what you just said. There is no way of knowing how slowing down affected the ride unless we fly thru the same (theoretically and practically impossible) pocket of turbulence 2 times at two different airspeeds.

No question, at optimum cruise altitudes we cant slow down significantly. Sometimes not at all.
In my airplane, if we fly at optimum altitude .80 above FL430, we are already at Vref or very close to it so there is no margin to slow down anyway.

However, if you depart Shanghai to Tokyo, Chinese will keep you at FL260 for 250nm. That is where we have an option to fly 320 knots or as slow as 240 knots at moderate weights and still have safe margins to stall (like Vref +70 or so).

@FE Hoppy

In my airplane turb pen speed is 240 below 10000ft and 270/.80 above
It doesnt take a rocket scientist to realise that it doesnt work like that: 240 knots at 9999ft and 270 knots at 10001ft.

For me personally, if I have a very good margin to Vref at current configuration and altitude with PAX, I will slow down below 270 KIAS for light to moderate and above. If it is ferry flight, I normally never slow down below Vb.

lol
you are taking the mickey right?
Would you rather have 3 numbers to remember or a graph?

[Sidestick_n_Rudder]

See the diagram of maneuver envelope AND gust envelope:
http://www.pilotfriend.com/FARS/6/1.jpg

As you can see, the load for vertical gust is linear with speed, whereas the maneuver loads are proportional to v-squared.

[G-V]

Thanks Sidestick.

Can you please post the source? Thanks

[underfire]

GV https://www.law.cornell.edu/cfr/text/14/23.333

[Outtahere]

Yeah i always get a chuckle when lads reduce from .8 to .79 or .78 in turbulence. Makes zero difference to the ride down the back.

But it does get you away from the overspeed for which you will be fined!

[Onceapilot]

Great posted illustrations from underfire and, Sidestick and rudder. :ok: Thanks.

[neila83]

OK, next time you see that nice Cu in front, enter it at 320. Then do an orbit and hit it at 250. Tell how you get on. Oh, and ask the cabin crew what they thought about the rides.

Are you saying you slow by 70 knots for a Cumulus? Your approaches must be interesting. Or is this a slightly bizarre hypothetical example with no relation to what's being discussed or real world practice?

The point being made is that slowing by M.02 is not going to make any meaningful difference in ride. The advice is there to protect the structure of the plane in severe turbulence, in everyday stuff it really isn't necessary for that purpose, and will make no practical difference to the ride.

[G-V]

Are you saying you slow by 70 knots for a Cumulus? Your approaches must be interesting. Or is this a slightly bizarre hypothetical example with no relation to what's being discussed or real world practice?


That was just an example.

Read the thread. Its not about slowing down from .82 to .80